Easy breezy

High humidity in the greenhouse is a concern for growers, as it increases the potential for the germination of fungal pathogen spores such as Botrytis and powdery mildews. This is a particular problem during the spring and fall months, as transpiration rates are elevated due to high light levels and temperature. Ventilation at this time of year is considerably less than during the summer.

Of the methods to reduce moisture, ventilation is the most common and usually the best. On sunny days, transpiration of moisture from leaf surfaces and evaporation from soil surfaces increases due to the warm temperature. This warm air holds moisture in the vapor form. At night when the thermostat is set lower, the air cools, reaching the dewpoint. Condensation occurs and water droplets are formed on cooler surfaces such as leaf surfaces and glazing. For example, when the greenhouse atmosphere is at 90 percent relative humidity and 60°F, condensation occurs when the leaf temperature drops to 57°F.

Going inside-out
One of the most effective methods of controlling humidity is to exhaust the moist air and replace it with cooler, dryer outside air. The temperature of the air greatly affects its capacity to hold water vapor. At 70°F, air will hold twice as much moisture as air at 50°F. In the range of temperatures encountered in a greenhouse, for every 20°F rise in dry bulb temperature, the water-holding capacity of the air doubles and relative humidity is reduced by one-half.

Applying this principle to the problem of reducing humidity, we see that by exhausting the humid air in the greenhouse and replacing it with cool, dry outside air (which, when warmed, will absorb more moisture), we can lower humidity.

Considering costs
In older glasshouses, air exchange was accomplished by infiltration through the glass laps. With today’s tight plastic and glass-covered houses built to retain heat, air exchange has to be accomplished through vents and fans. As high humidity usually occurs in the late afternoon to early morning when temperatures are cool, ventilation may require heating the replacement air to maintain the temperature in the greenhouse. Growers hesitate to do this, as they are concerned about increasing heating fuel use. In reality, the cost of doing this is relatively small.

Based on 1,000 sq ft of greenhouse floor area (approximately 10,000 cu ft of air), it would take 4,000 Btu of heat to raise the temperature of one volume of replacement air 20°F. At $3.00/gallon of fuel oil, or $2.00/gallon of propane, and 75 percent heater efficiency, the cost is about $0.11/cycle. With natural gas at $1.10/ccf, the cost is only $0.05. This is usually done about two or three times per hour during the evening after the sun goes down and again early in the morning at sunrise.
 

Control for this can be with a timer/time clock, humidistat or controller with a humidity sensor. With fan ventilation, it may be necessary to install a relay to lock out the furnace or boiler until the fan shuts off so that flue gases are not drawn into the greenhouse. In greenhouses with vents, cycling the vents to open and close is better than leaving them cracked open all night.

Other techniques that can supplement ventilation to reduce humidity include avoiding overwatering, which adds moisture to floors and walkways; using a wetting agent on glazing to prevent condensate dripping and increase light levels; employing bottom heat to create air movement up through the plants and warm the leaf surfaces; and using horizontal air flow (HAF), which circulates air past the leaf surfaces, thus warming them and removing high-humidity air.